Flexible spine stabilization system

ABSTRACT

A spine stabilization system includes a flexible member attachable to a portion of the spinal column. The member includes components that are oriented and function similar to the natural fiber orientation of the anterior longitudinal ligament and annulus tissue. The use of components resist loading applied by extension and rotation of the spine, while the flexibility of the member does not subject it to the compressive loading of the spinal column segment to which it is attached.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of the filing date ofProvisional Patent Application Ser. No. 60/272,102 filed on Feb. 28,2001.

BACKGROUND

The present invention relates to orthopedic implants, and moreparticularly, to flexible spinal stabilization systems.

Interbody fusion device, artificial discs, interbody spacers and otherdevices have been inserted in a spinal disc space or engaged to avertebral body. For example, as shown in FIG. 1, a pair of interbodyfusion devices I1 and I2 are inserted into an intradiscal space betweenthe L5 and S1 levels of the spinal column. Aorta A1 and vena cava A2along with other tissue and vasculature also extend along the anterioraspect of the spinal column. As shown in FIG. 2, the anteriorlongitudinal ligament AL extends along the anterior portion of the discspace. The disc space is surrounded by annulus fibrosus or annulusfibers AF. Insertion of implants I1 and I2 into the disc space can befacilitated by the removal of all or a portion of the anteriorlongitudinal ligament AL and the annulus fibers AF.

In order to stabilize the spinal column, it is known to secure a rigidmetal construct to each of the vertebral bodies on either side of thespinal disc space after inserting devices or performing surgicalprocedures in the disc space or on the vertebral bodies. For example, arigid metal plate can be placed along the anterior aspect of thevertebrae and secured to the L5 and S1 levels after insertion ofimplants I1 and I2 into the disc space therebetween. In another example,a rigid rod or plate can be secured to the posterior portions ofvertebrae V1 and V2 after anterior insertion of implants I1 and I2.

While rigid metal constructs provide adequate load resistance, there canbe drawbacks, such as the intrusion of the construct into the adjacenttissue and vasculature, stress shielding, multiple surgeries forinstallation, and fatigue. What are needed are systems that do notrequire posterior hardware to support the spinal column or rigidanterior, antero-lateral, or lateral plates and constructs. The systemsshould be resistant to fatigue, stress shielding and tensile androtational loads that are typically applied to the spinal column. Thepresent invention is directed toward meeting these needs, among others.

SUMMARY OF THE INVENTION

The present invention is directed to spine stabilization systems thatare flexible and include components that replicate, substitute and/oraugment the natural fiber orientation of the anterior longitudinalligament and annulus tissue. The components resist loading applied byextension and rotation of the spine, while the flexibility of thecomponents does not subject them to the compressive loading of thespinal column segment to which it is secured.

DESCRIPTION OF THE FIGURES

FIG. 1 is an elevational view of a spinal column segment having a pairof interbody fusion devices inserted into a disc space.

FIG. 2 is a plan view of a disc space of a spinal column segment and thesurrounding tissue and vasculature.

FIG. 3 is a perspective view a spinal column segment with its associatedligaments.

FIGS. 4(a) and 4(b) illustrate various features of a spinal disc space.

FIGS. 5(a) and 5(b) illustrate various structural properties of theannulus fibrosous.

FIG. 6 is a flexible spine stabilization system according to oneembodiment secured to a spinal column segment.

FIGS. 7(a) through 7(c) show various components of the system of FIG. 6.

FIG. 8 is an elevational view of another embodiment flexible spinestabilization system secured to a spinal column segment.

FIG. 8a is an enlarged detail view of a portion of the system of FIG. 8.

FIG. 9 is elevational view of yet another embodiment flexible spinestabilization system secured to a spinal column segment.

FIG. 10(a) is an elevational view of a further embodiment flexible spinestabilization system.

FIG. 10(b) is the system of FIG. 10(a) secured to a spinal columnsegment.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the illustrated embodiments andspecific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any such alterations and furthermodifications of the invention, and any such further applications of theprinciples of the invention as illustrated herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

The present invention is directed to flexible spine stabilizationsystems for placement on the anterior aspect of the vertebrae of aspinal column segment. It is contemplated the systems may also be placedon the antero-lateral aspect or the lateral aspect of the vertebrae. Itis also contemplates that the systems can extend across one or morevertebral levels. The systems are configured to replicate, substituteand/or augment the structure and function of the natural occurringfibers that protect the intervertebral disc space. It is furthercontemplated that the systems can be used in lieu of the placement of arigid anterior plate across one or more disc spaces after insertion ofan interbody fusion device into the disc space. It is also contemplatedthat the systems can be used in non-interbody fusion procedures.

The spine stabilization systems include a member having a first set ofone or more components oriented generally in the direction of theannulus fibers, and a second set of components oriented generally in thedirection of the fibers of the anterior longitudinal ligament. The useof components having such orientations provides resistance to loadingapplied by extension, lateral bending, and rotation of the spine. Theflexibility of the member does not subject it to stress shielding causedby the compressive loading of the spinal column segment to which it issecured. Thus, the spine stabilization systems of the present inventionreplicate, augment and/or substitute the load resistant propertiescapabilities of the annulus fibers and anterior longitudinal ligament.

Further description of flexible spinal stabilization systems is providedafter the following discussion of the anatomical features of the annulusand anterior longitudinal ligament. Referring now to FIG. 3, a spinalcolumn segment including an upper vertebra V1 and lower vertebra V2 isillustrated. The spinal column segment includes anterior longitudinalligament AL extending along the anterior aspects of vertebra V1 andvertebra V2 and across the intervertebral disc space between vertebraeV1 and V2. The nucleus of the disc space is protected by annulus fibersAF. The spinal column segment further includes posterior longitudinalligament PL, intertransverse ligament IL, facet capsular ligament FC,interspinous ligament ISL, superspinous ligament SSL, and ligamentumflavum LF. The spine is designed in such a way that when the functionalspinal unit is subjected to different complex force and torque vectors,the individual ligaments provide tensile resistance to external loads bydeveloping tension. When a surgical procedure interrupts or removes aportion of these ligaments, the ability of the spinal unit to resistthese complex force and torque vectors is compromised.

The anterior longitudinal ligament AL is an uni-axial structure, and ismost effective in carrying loads along the direction in which the fibersrun. The anterior longitudinal ligament AL has a fibrous tissuestructure that arises from the anterior aspect of the basioccipital andis attached to the atlas and the anterior surfaces of all vertebrae,down to and including a part of the sacrum. It is firmly attached to theedges of the vertebral bodies, but is less firmly affixed to the annulusfibers AF. The width of the anterior longitudinal ligament AL diminishesat the level of disc and is narrower and thicker in the thoracic regionof the spine. The fibers of the anterior longitudinal ligament run alongthe length of the spinal column in the direction of the central spinalcolumn axis and transverse to axial plane H. The fibers of the anteriorlongitudinal ligament AL are much like rubber bands in that they readilyresist tensile forces, but buckle when subjected to compressive forces.

Referring now to FIGS. 4(a) and 4(b), further properties of the spinaldisc space D will be discussed. The nucleus pulposus N of the disc spaceis surrounded by the annulus fibrosus AF. The annulus fibrosus AFincludes a number of concentric laminated bands, designated as annuluslaminates AN1, AN2, AN3, and AN4 as shown in FIG. 4(a). As shown furtherin FIG. 4(b), these annulus laminates AN have fibers oriented either +30degrees or −30 degrees with respect to axial plane H of the spinalcolumn. Stated another way, the AN fibers are oriented about +60 degreesor about −60 degrees with respect to the fibers of the anteriorlongitudinal ligament AL. The fibers of the adjacent annulus laminatesAN are non-orthogonal with respect to axial plane H and to one another,forming a criss-cross pattern as shown by AN1 and AN2 of FIG. 2(b). Wheninterbody fusion devices or other implants are inserted between thevertebrae, or when the disc space is access for surgical procedures, theresection, removal and other cutting required of the annulus laminatesAN and anterior longitudinal ligament AL disrupts fiber orientation andcontinuity.

Referring now to FIGS. 5(a) and 5(b), the tensile properties andstrength properties of the annulus fibrosis will now be discussed. Theannulus fibers AF resist tensile forces and torque or rotational forcesapplied to the spinal column segment. In FIG. 5(a), the tensilestiffness of the annulus AF in different directions is shown. Thestiffness of annulus fibrosus AF is highest along a direction oriented15 degrees with respect to axial plane H. In FIG. 5(b), the strength ofthe annulus AF has been found to be greatest along a direction oriented30 degree with respect to horizontal axis H extending through the discspace. It has further been found that the strength of the annulus AF isthree times greater along this 30 degree axis as compared to thestrength along axial plane H.

Referring now to FIG. 6, a spine stabilization system 20 is illustrated.System 20 is secured to vertebra V1 and vertebra V2 and extends acrossthe disc space D. It is contemplated that one or more interbody fusiondevices, interbody spacers, artificial discs or other implant can beinserted into disc space D in a procedure prior to attachment of system20 to vertebrae V1 and V2. It is also contemplated that a surgicalprocedure can be performed in or on disc space D such as, for example,removal of discal material to repair a herniated disc. In suchprocedures, the excision or disruption to the annulus fibrosus AF andthe anterior longitudinal ligament AL compromises the ability of thesespinal structures to resist extension, torsion, and lateral bending ofthe spinal column. System 20 can restore this ability and can cover theentry made through the annulus fibrosis AF and anterior longitudinalligament AL, preventing devices inserted into the disc space frombacking out or protruding through the created opening.

System 20 includes diagonal components and vertical components. Thediagonal components can be oriented in the range of 15 degrees to 60degrees with respect to axial plane H when the devices are secured tothe vertebrae. The vertical components extend generally perpendicular toaxial plane H. Stated another way, a first set of diagonal componentsextends at an angle A1 in the range of +30 degrees to +75 degreesrelative to the vertical components, and a second set of diagonalcomponents extends transverse to the first set and at an angle A2 in therange of −30 degrees to −75 degrees relative to the vertical components.In another form, the first set of diagonal components extends at anangle A1 in the range of +45 degrees to +60 degrees relative to thevertical components, and the second set of diagonal components extendstransverse to the first set and at an angle A2 in the range of −45degrees to −60 degrees relative to the vertical components. In a furtherform, the first set of diagonal components extends at an angle A1 ofabout +60 degrees relative to the vertical components, and the secondset of diagonal components extends transverse to the first set and at anangle A2 of about −60 degrees relative to the vertical components.

Referring now to FIGS. 7(a) through 7(c), there are shown variouscomponents of system 20. System 20 includes a member extending betweenthe first and second vertebrae V1 and V2. The member includes a firstlayer 30 which has a number of vertically oriented components 34 and anumber of horizontally oriented components 32. Components 32, 34 areinterwoven or otherwise attached to form a grid-like or mesh patternhaving a number of square apertures therethrough. The member furtherincludes a second layer 35 as shown in FIG. 7(b) having a number offirst diagonal components 36 and a number of second diagonal components38. Diagonal components 36 and 38 are interwoven or otherwise attachedto form a grid-like or mesh pattern having a number of diamond-shapedapertures. Each layer 30, 35 is flexible in compression yet inelastic orsubstantially inelastic to resist tensile loading.

The individual components 32, 34, 36, 38 of each layer can be made froma small diameter or cross-section wire, fiber, rod, strand or otherelongated component. As shown in FIG. 7(c), first layer 30 is placed ontop of second layer 35 and secured thereto via grommets 26. Othersecurement devices and techniques are also contemplated, including, forexample, other fasteners such as rivets, clamps, cables, sutures,staples, and hooks; and chemical/thermal bonding, such as welding oradhesives. Other embodiments contemplate that layers 30, 35 are notengaged to one another, but rather simply placed on top of one anotherwhen secured to the spinal column.

A number of openings 24 are formed through the layers to accommodatefasteners 22. In the illustrated embodiment, four such openings areformed, with one opening positioned adjacent each corner. Eyelets orgrommets 26 extend around each hole 24. As shown in FIG. 6, boneengaging fasteners 22 can be placed through corresponding ones of theholes 24 to secure layers 30, 35 to vertebrae V1 and V2. The corners oflayers 30, 35 can be rounded to provide gradual transitions between thevertical and horizontal edges thereof. Other embodiments contemplate oneopening 24 at each vertebra, and further embodiments contemplate morethan two openings 24 at each vertebra. Other embodiments alsocontemplate that no openings are provided, but rather the fastenersextend directly through the layers of material.

The vertically extending components 34 of first layer 30 are orientedand function in a manner similar to the longitudinal fibers of theanterior longitudinal ligament AL. Diagonal components 36, 38 of secondlayer 35 are non-orthogonally oriented with respect to the verticalcomponents 34, and are oriented and function similar to annulus fibersAF. Thus, system 20 replaces, substitutes, and/or augments the functionof the naturally occurring fibers of the anterior longitudinal ligamentAL and the annulus fibrosis AF. The orientation of the components ofsystem 20 are such that forces caused by extension, rotation, andlateral bending of the spinal column are resisted in a manner the sameas or similar to the natural occurring anatomical structures provided toresist such forces.

Referring to FIGS. 8 and 8a, there is provided a spine stabilizationsystem 40 having a layer 46. Layer 46 includes a number of interwovencomponents that include at least longitudinally and diagonally orientedcomponents. In the illustrated embodiment, there are provided firstdiagonal components 47, second diagonal components 48, verticalcomponents 49, and horizontal components 45. The first and seconddiagonal components 47, 48 are oriented and function similar to thefibers of the annulus fibrosis AF, and the vertical components 49 areoriented and function similar to the anterior longitudinal ligament AL.Small apertures are provided through layer 46 between its components.Eyelets or grommets 44 each surround a corresponding one of the holesprovided through layer 46. Bone engaging fasteners 42 extend throughthese holes to attach system 40 to vertebra V1 and vertebra V2.

Referring now to FIG. 9, there is shown another embodiment spinestabilization system 50. The components of system 50 are diagonally andvertically oriented to replicate the orientation of the fibers of theanterior longitudinal ligament AL and annulus fibers AF. System 50includes a member formed by a layer 56 made from components interwovensuch that there are no apertures through layer 56, providing asubstantially solid wall across disc space D. Layer 56 includes a numberof holes formed therethrough surrounded by grommets or eyelets 54.Fasteners 52 extend through the holes in order to secure the flexiblesystem 50 to vertebra V1 and V2 across the annulus fibrosis AF.

Referring now to FIG. 10(a), another embodiment spine stabilizationsystem 60 is illustrated. System 60 includes member having a firstvertical component 66 and a second vertical component 68. Verticalcomponents 66 and 68 are interconnected by a first diagonal component 70and a second diagonal component 72. As shown in FIG. 10(b), firstdiagonal component 70 extends from a right lateral side of vertebralbody V2 across the sagittal plane L to a left lateral side of vertebralbody V1. Second diagonal component 72 extends from the left lateral sideof vertebral body V2 across thee sagittal plane L to the right lateralside of vertebral body V1. The first vertical component 66 extends froma left lateral side of vertebral body V2 to the left lateral side ofvertebral body V1 offset to one side of sagittal plane L. Secondvertical component 68 extends from the right lateral side of vertebralbody V2 to the right lateral side of vertebral body V1 offset to theother side of sagittal plane L. In each corner of system 60 there isformed a hole 64 for receiving a fastener 62 to attach system 60 tovertebrae V1 and V2.

The vertically oriented components 66, 68 are oriented to replace,substitute and/or augment the structure function of the anteriorlongitudinal ligament AL and resist at least extension forces applied tothe spinal column segment. The diagonal components 70, 72 arenon-orthogonally oriented with respect to the vertically orientedcomponents 66, 68 and resist at least rotational and torque forces onthe spinal column segment. Thus, diagonal components 70, 72 are orientedand function similar to the fibers of the annulus fibrosis AF.

The components of the flexible spinal stabilization systems can be madefrom one or a combination of metal material, polymeric material, ceramicmaterial, shape memory material, and composites thereof. The componentsof the systems can also be coated or impregnated with anti-adhesivematerial that will prevent tissue and vasculature from attachingthereto. The components of the systems can be provided in multiplelayers each including one or more components with the desiredorientation and placed one on top of the other, or the components can beprovided in a single interwoven layer that includes the desiredcomponent orientation.

In one specific embodiment, the components are made from metal wire meshof suitable tensile strength and which is not subject to substantialcreep deformation or in vitro degradation. It is contemplated that thewire can be made from stainless steel, cobalt-chrome alloy, titanium,titanium alloy, or nickel-titanium, among others.

In another specific embodiment, the components are made from a softfiber material. Soft fiber material can include polymeric material, suchas SPECTRA fiber, nylon, carbon fiber and polyethylene, among others.Examples of suitable metal polymers include DACRON and GORE-TEX. Oneadvantage provided by a soft fiber design is that the risk of tissue andvascular injury is further mitigated by reducing the abrasive qualitiesof the component material. A further advantage is that the componentsfibers can be radiolucent, allowing radiographic imaging for assessmentand monitoring of the disc space and any implant, fusion devices, orartificial disc inserted therein. Another advantage is that somepolymeric materials, such as spectra fiber, can be stronger than metalsand less susceptible to fatigue or creep.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatthe preferred embodiments have been shown and described, and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

What is claimed is:
 1. A stabilization system for a spinal column segment with an anterior longitudinal ligament having a first fiber orientation extending in the direction of a central spinal column axis, the spinal column segment having annulus tissue with a second fiber orientation extending at an acute angle to the first fiber orientation and a third fiber orientation extending at an acute angle to the first fiber orientation and transverse to the second fiber orientation, the system comprising: a flexible member attachable to the bodies of at least first and second vertebrae of the spinal column segment, said member including at least one first component extending in the direction of the first fiber orientation, at least one second component extending generally in a direction corresponding to the second fiber orientation, and at least one third component extending generally in a direction corresponding to the third fiber orientation.
 2. The system of claim 1, wherein said at least one first component includes a number of first components forming a first mesh layer with a number of fourth components extending transverse to said number of first components.
 3. The system of claim 2, wherein said at least one second component includes a number of second components and said at least one third component includes a number of third components, said number of second and third components forming a second mesh layer.
 4. The system of claim 3, wherein said first mesh layer and said second mesh are place one on top of the other for attachment to the bodies of the first and second vertebrae.
 5. The system of claim 3, wherein said first mesh layer is interwoven with said second mesh layer.
 6. The system of claim 1, further comprising fasteners for attaching the member to each of the first and second vertebrae.
 7. The system of claim 1, wherein said at least one first component, said at least one second component, and said at least one third component are inelastic.
 8. The system of claim 1, wherein said at least one first component, said at least one second component, and said at least one third component include a tissue anti-adhesion material.
 9. The system of claim 1, wherein said member defines a plurality of openings therethrough.
 10. The system of claim 1, wherein said at least one second component forms a first angle in the range of +30 to +75 degrees with said at least one first component and said at least one third component forms a second angle in the range of −30 to −75 degrees with said at least one first component.
 11. The system of claim 10, wherein said first angle is in the range of +45 to +60 degrees and said second angle is in the range of −45 to −60 degrees.
 12. The system of claim 10, wherein said first angle is about +60 degrees and said second angle is about −60 degrees.
 13. The system of claim 1, wherein said member is attachable to an anterior face of said first and second vertebrae.
 14. The system of claim 1, further comprising at least one spinal fusion implant between the first and second vertebrae.
 15. A stabilization system for a spinal column segment with an anterior longitudinal ligament having a first fiber orientation extending in the direction of a central spinal column axis, the spinal column segment having annulus tissue with a second fiber orientation extending at an acute angle to the first fiber orientation and a third fiber orientation extending at an acute angle to the first fiber orientation and transverse to the second fiber orientation, the system comprising: a flexible member attachable to the bodies of first and second vertebrae of the spinal column segment, said member including at least one layer formed by a number of first components extending in the direction of the first fiber orientation, a number of second components extending generally in a direction corresponding to the second fiber orientation, and a number of third components extending generally in a direction corresponding to the third fiber orientation.
 16. The system of claim 15, wherein said layer further comprises a number of fourth components extending transverse to said number of first components.
 17. The system of claim 15, wherein said first components, said second components and said third components are interwoven with one another.
 18. The system of claim 15, further comprising fasteners for securing the member to each of the first and second vertebrae.
 19. The system of claim 15, wherein said first components, said second components, and said third components are inelastic.
 20. The system of claim 15, wherein said member defines a plurality of apertures therethrough.
 21. The system of claim 15, wherein said second components form a first angle in the range of +30 to +75 degrees with said first components and said third components form a second angle in the range of −30 to −75 degrees with said first components.
 22. The system of claim 15, wherein said first angle is in the range of +45 to +60 degrees and said second angle is in the range of −45 to −60 degrees.
 23. The system of claim 15, wherein said first angle is about +60 degrees and said second angle is about −60 degrees.
 24. The system of claim 15, wherein said at least one layer is a wire mesh made from a material selected from the group consisting of: stainless steel, cobalt-chrome alloy, titanium, titanium alloy, nickel-titanium, and shape memory metal alloy.
 25. The system of claim 15, wherein said at least one layer is a mesh made from a material selected from the group consisting of: polymers and metal polymers.
 26. A stabilization system for a spinal column segment with an anterior longitudinal ligament having a first fiber orientation extending in the direction of a central spinal column axis, the spinal column segment having annulus tissue with a second fiber orientation extending at an acute angle to the first fiber orientation and a third fiber orientation extending at an acute angle to the first fiber orientation and transverse to the second fiber orientation, the system comprising: a flexible member attachable to the bodies of first and second vertebrae of the spinal column segment, said member including: at least one first layer formed by a number of first components extending in the direction of the first fiber orientation; and at least one second layer formed by a number of second components extending generally in a direction corresponding to the second fiber orientation, and a number of third components extending generally in a direction corresponding to the third fiber orientation.
 27. The system of claim 26, wherein said first layer and said second layer are secured to one another.
 28. The system of claim 26, wherein said at least one first layer further includes a number of fourth components extending transversely to said number of first components.
 29. The system of claim 26, wherein said member has a number of holes formed through said first and second layers sized to receive fasteners for attaching said member to the first and second vertebrae.
 30. The system of claim 29, wherein each of said holes includes a grommet extending therearound.
 31. The system of claim 30, wherein said grommets secure said first layer to said second layer.
 32. The system of claim 26, wherein said member defines a plurality of apertures therethrough.
 33. The system of claim 26, wherein said at least one second component forms a first angle in the range of +30 to +75 degrees with said at least one first component and said at least one third component forms a second angle in the range of −30 to −75 degrees with said at least one first component.
 34. The system of claim 33, wherein said first angle is in the range of +45 to +60 degrees and said second angle is in the range of −45 to −60 degrees.
 35. The system of claim 33, wherein said first angle is about +60 degrees and said second angle is about −60 degrees.
 36. The system of claim 26, wherein each of said first layer and said second layer is a wire mesh made from a material selected from the group consisting essentially of: stainless steel, cobalt-chrome alloy, titanium, titanium alloy, nickel-titanium, and shape memory metal alloy.
 37. A method for stabilizing a portion of a spinal column including an anterior longitudinal ligament and an annulus having a first portion of fibers oriented in a first direction transverse to the anterior longitudinal ligament and a second portion of fibers oriented in a second direction transverse to the anterior longitudinal ligament and transverse to the first portion of fibers, the method comprising: providing at least one first component oriented in the direction of the anterior longitudinal ligament; providing at least one second component oriented in the direction of the first portion of the annulus fibers; providing at least one third component oriented in the direction of the second portion of the annulus fibers; and securing the at least one first component, the at least one second component and the at least one third component to first and second vertebrae of the spinal column.
 38. The method of claim 37, further comprising applying a tissue non-adhesive coating to the at least one first component, the at least one second component and the at least one third component.
 39. The method of claim 37, wherein the at least one first component, the at least one second component and the at least one third component are interwoven in a mesh layer.
 40. The method of claim 37, further comprising providing at least one fourth component oriented transversely to the at least one first component.
 41. The method of claim 40, further comprising: providing the at least one first component and the at least one fourth component in a first mesh layer; providing the at least one second component and the at least one third component in a second mesh layer; and placing the first mesh layer and the second mesh layer one on top of the other.
 42. The method of claim 37, further comprising inserting an implant between the first and second vertebrae before securing the at least one first component, the at least one second component and the at least one third component to the first and second vertebrae. 